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ratio (1/6) of these numbers. If this ratio were zero, then two organisms would encode exactly the same enzymes. If it were 1/2, then half of the enzymes that one organism can produce would also be produced by the other organism. If the ratio were equal to 1, then the first organism wouldn’t be able to produce a single enzyme that the second organism can produce—their metabolisms would be maximally different. Those ratios, ranging from 0 to 1, reflect the difference between the enzymatic portfolios of two organisms, but that’s a little unwieldy to write over and over again—better to replace it with the symbol
D
for
difference
or
distance
. 28
    Unspeakably tedious as comparing genotypes for hundreds of bacteria—each encoding more than a thousand reactions—would be on pencil and paper, my trusted computer can finish it in the blink of an eye. When I asked it to calculate
D
for hundreds of pairs of bacteria, I was surprised to see—although their highly diverse genomes should have warned me—that even closely related organisms had highly diverse metabolic texts. Thirteen different strains of
E. coli
differed in more than 20 percent of their enzymes. 29 An average pair of microbes differed in more than half of them. 30 I had also suspected that bacteria living in the same environment—the soil, for example, or the ocean—might encounter similar nutrients and thus have similar metabolic texts. Wrong. Their metabolic texts were just as diverse, with a
D
just as different as that from bacteria living in different environments.
    This exercise underscores the staggering scale at which nature experiments through gene shuffling. Everywhere on this planet, a relentless shuffling and mixing and recombining of genes takes place. Wherever microbial life occurs, in the depth of the oceans and on arid mountaintops, in scalding hot springs and on frigid glaciers, in fertile soils and desiccated deserts, inside and around our bodies, life is experimenting with every conceivable combination of new genes, rereading, editing, and rejuggling its metabolic texts without pause, yielding an enormous and still growing diversity of metabolisms.

     
    Without readers, a book is a bundle of cellulose sheets with meaningless ink stains. Likewise, a text in the metabolic library needs to be read to reveal its meaning: the metabolic phenotype that determines which fuels an organism can use, and which molecules it can manufacture. We think of a phenotype as something we can see, and many metabolic phenotypes are plain as daylight. They include the melanins that protect our skin against radiation, that camouflage a lion’s fur, and that color the ink of an octopus. All of them are molecules synthesized by metabolism. And so are the various pigments that dye tree leaves, lobsters, flowers, and chameleons, whether for defense, courtship, or sometimes for no good reason at all. 31 But metabolic phenotypes do not end at this visible surface. They extend to depths that are hidden from our eyes yet visible to chemical instruments—and to natural selection. Their most important role is to ensure viability itself, which boils down to the ability to synthesize sixty-odd molecules very different from those pretty pigments—they are the essential biomass molecules I mentioned in chapter 2. Viability, viewed as the phenotypic meaning of a genotypic text, is like the simple moral of a complex story, or like a brutally straightforward court judgment: If you can’t make all essential biomass molecules, your sentence is death, and it is carried out immediately. Organisms with a mutation that has compromised the ability to synthesize essential molecules don’t just fail to live long enough to reproduce. They don’t live at all.
    To grasp this phenotypic meaning—viability or death—we need to read an organism’s metabolic genotype. This is a tall order, not only because the meaning of a text is so much more complex than the text itself—to understand